In the recent drive for higher integration and operating speeds in LSI devices, the pattern rule is made drastically finer. The background supporting such a rapid advance is a reduced wavelength of the light source for exposure. The change-over from i-line (365 nm) of a mercury lamp to shorter wavelength KrF laser (248 nm) enabled mass-scale production of dynamic random access memories (DRAM) with an integration degree of 64 MB (processing feature size≦0.25 μm). To establish the micropatterning technology necessary for the fabrication of DRAM with an integration degree of 256 MB and 1 GB or more, the lithography using ArF excimer laser (193 nm) is under active investigation. The ArF excimer laser lithography, combined with a high NA lens (NA≦0.9), is considered to comply with 65-nm node devices. For the fabrication of next 45-nm node devices, the F2 laser lithography of 157 nm wavelength became a candidate. However, because of many problems including a cost and a shortage of resist performance, the employment of F2 lithography was postponed. ArF immersion lithography was proposed as a substitute for the F2 lithography. Efforts have been made for the early introduction of ArF immersion lithography (see Proc. SPIE, Vol. 4690, xxix, 2002).
In the ArF immersion lithography, the space between the projection lens and the wafer is filled with water and ArF excimer laser is irradiated through the water. Since water has a refractive index of 1.44 at 193 nm, pattern formation is possible even using a lens with NA of 1.0 or greater. Theoretically, it is possible to increase the NA to 1.44. The resolution is improved by an increment of NA. A combination of a lens having NA of at least 1.2 with ultra-high resolution technology suggests a way to the 45-nm node (see Proc. SPIE, Vol. 5040, p 724, 2003).
Several problems arise when a resist film is exposed in the presence of water. For example, the acid once generated from a photoacid generator and a basic compound added to the resist can be partially leached in water. As a result, pattern profile changes and pattern collapse can occur. It is also pointed out that water droplets remaining on the resist film, though in a minute volume, can penetrate into the resist film to generate defects.
These drawbacks of the ArF immersion lithography may be overcome by providing a protective coating between the resist film and water to prevent resist components from being leached out and water from penetrating into the resist film (see the 2nd Immersion Workshop, Resist and Cover Material Investigation for Immersion Lithography, 2003).
With respect to the protective coating on the photoresist film, a typical antireflective coating on resist (ARCOR) process is disclosed in JP-A 62-62520, JP-A 62-62521, and JP-A 60-38821. The ARCs are made of fluorinated compounds having a low refractive index, such as perfluoroalkyl polyethers and perfluoroalkyl amines. Since these fluorinated compounds are less compatible with organic substances, fluorocarbon solvents are used in coating and stripping of protective coatings, raising environmental and cost issues.
Other resist protective coating materials under investigation include water-soluble or alkali-soluble materials. See, for example, JP-A 6-273926, Japanese Patent No. 2,803,549, and J. Photopolymer Sci. and Technol., Vol. 18, No. 5, p 615, 2005. Since the alkali-soluble resist protective coating material is strippable with an alkaline developer, it eliminates a need for an extra stripping unit and offers a great cost saving. From this standpoint, great efforts have been devoted to develop water-insoluble resist protective coating materials, for example, resins having alkali-soluble groups such as fluorinated alcohol, carboxyl or sulfo groups on side chains. See WO 2005/42453, WO 2005/69676, JP-A 2005-264131, JP-A 2006-133716, and JP-A 2006-91798.
Required of the resist protective coating materials are not only the ability to prevent the generated acid and basic compound in the photoresist film from being leached out in water and to prevent water from penetrating into the resist film, but also such properties as water repellency and water slip. Of these properties, water repellency is improved by introducing fluorine into the resin and water slip is improved by combining water repellent groups of different species to form a micro-domain structure, as reported, for example, in XXIV FATIPEC Congress Book, Vol. B, p 15 (1997) and Progress in Organic Coatings, 31, p 97 (1997).
One exemplary polymer exhibiting high water slip and water repellency is a fluorinated ring-closing polymerization polymer having hexafluoroalcohol pendants. It is reported in Proc. SPIE, Vol. 6519, p 651905 (2007) that this polymer is further improved in water slip by protecting hydroxyl groups on its side chains with acid labile groups.
Although the introduction of fluorine into resins is effective not only for improving water repellency, but also for improving water slip properties as demonstrated by sliding angle, receding contact angle or the like, excessive introduction of fluorine results in resins with a greater surface contact angle following alkaline development. In the current technology, those defects so called “blob defects” that occur on the resist film surface (especially in the unexposed area) after development are regarded problematic. A tendency is known that a resist film having higher water repellency suffers from more blob defects. Accordingly, introducing extra fluorine into resins for the purpose of enhancing water repellency and water slip increases a likelihood of blob defects occurring.
It is believed that blob defects are caused by water droplets remaining on the resist film surface after development. The internal energy of a water droplet on a resist film increases in the spin drying step and reaches the maximum when the water droplet leaves the resist film surface. At the same time as the water droplet leaves the resist film surface, the resist film surface is damaged by that energy, which is observable as blob defects.
The internal energy of a water droplet on a resist film is higher as the surface becomes more water repellent. When a protective coating with higher water repellency is disposed on a resist film, the resist surface has a greater contact angle due to intermixing between the resist film and the protective coating, increasing a likelihood of blob defects occurring. This indicates that for the purpose of suppressing the occurrence of blob defects, the surface contact angle after development must be reduced in order to reduce the internal energy of a water droplet.
Application of a more hydrophilic resist protective coating may be effective for reducing the surface contact angle after development. However, such a protective coating provides a smaller receding contact angle, which interferes with high-speed scanning and allows water droplets to remain after scanning, giving rise to defects known as water marks. A resist protective coating having carboxyl or sulfo groups is proposed in U.S. Pat. No. 7,455,952 (JP-A 2006-91798). Since both carboxyl and sulfo groups are highly hydrophilic, water repellency and water slip worsen.
It is then proposed to form a protective coating from a blend of a first polymer having sulfo groups and a second polymer having highly water repellent hexafluoroalcohol groups such that the second polymer having hexafluoroalcohol groups is segregated at the surface of the protective coating and the first polymer having sulfa groups is segregated at the interface with the underlying resist. See 4th Immersion Symposium RE-04 New Materials for surface energy control of 193 nm photoresists, Dan Sander et al. Although this protective coating is effective in reducing blob defects, the resist pattern suffers from film slimming after development because sulfo groups bind with an amine component in the resist so that the amine component becomes depleted near the resist surface. There exists a desire for a protective coating which prevents film slimming in order to produce a rectangular profile pattern and renders more hydrophilic the resist surface after development in order to inhibit blob defects.
The resist protective coating materials discussed above are needed not only in the ArF immersion lithography, but also in the electron beam (EB) lithography. When EB lithography is performed for mask image writing, it is pointed out that the resist changes its sensitivity due to evaporation of the acid generated during image writing, evaporation of vinyl ether produced by deprotection of acetal protective groups, or the like, as discussed in JP-A 2002-99090. It is then proposed to suppress resist sensitivity variation by applying a protective coating material to form a barrier film on top of a resist film.